1. Field of the Invention
The present invention relates to fuel vapor recovery systems for gas vehicle, particularly relates to fuel vapor recovery systems each having a separation module capable of separating fuel vapor from air.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 2010-116872 discloses a conventional fuel vapor recovery system having an adsorbent canister that can trap fuel vapor, a negative pressure supplier that provides negative pressure to the adsorbent canister in order to remove the fuel vapor from the adsorbent canister, and a separation module that has a separation membrane capable of separating the fuel vapor from air. The fuel vapor recovery system has a vacuum pump as negative pressure supplier. When the vacuum pump operates, air is introduced into the adsorbent canister in order to remove the fuel vapor from the adsorbent canister and then mixture of the fuel vapor and air is introduced into the separation module and is separated into the fuel vapor and air. In detail, the vacuum pump is provided between the adsorbent canister and the separation module. Air remaining in the separation module, i.e., air that has not passed through the separation membrane, is introduced into the adsorbent canister in order to purge the adsorbent canister. Here, while purging the adsorbent canister, fuel vapor vaporized in the fuel tank is not introduced into the adsorbent canister or the like.
In such system, the separation membrane divides an inner space of the separation module into a receiving chamber fluidly communicated with an inlet for receiving fuel vapor containing gas and a permeation chamber fluidly communicated with an outlet for discharging separated fuel vapor. With regard to the separation module having the separation membrane, it is known that when difference between partial pressures of the fuel vapor in the receiving chamber and the permeation chamber across the separation membrane becomes larger, separation efficiency of the separation membrane (amount of fuel vapor separated per unit of time) becomes higher. Thus, in a condition where the fuel vapor presents in the receiving chamber, when concentration of fuel vapor in the permeation chamber becomes lower, the separation efficiency of the separation membrane becomes higher. In the separation module disclosed in Japanese Laid-Open Patent Publication No. 2010-116872, the vacuum pump applies positive pressure to the receiving chamber, however pressure in the permeation chamber is not controlled. Thus, concentration (partial pressure) of the fuel vapor in the permeation chamber is constantly high, so that it is difficult to efficiently separate the fuel vapor from air. Accordingly, there has been a need for improved fuel vapor recovery system.